1. Field of the Invention
This invention generally relates to integrated circuit (IC) semiconductor processes and, more particularly, to a process for transferring single-crystal silicon thin-films to a glass substrate.
2. Description of the Related Art
The formation of high quality silicon (Si) on transparent substrates continues to be a problem for the IC semiconductor industry. Glass substrates are inexpensive and useful in display applications. However, glass is temperature sensitive and subject to degradation when exposed to temperatures exceeding 600° C. To form high speed or high current capacity thin-film transistors and other active components on a glass substrate, a single-crystal active Si layer would be desirable. However, amorphous thin-film Si can only be converted to a polycrystalline structure. As a compromise, lower process temperature polycrystalline Si thin-films are often used overlying a glass substrate.
An alternate approach is to form the single-crystal Si thin-film in a separate process, and subsequently transfer the Si film to a glass substrate. In one process, strained Si is first formed on a relaxed Silicon/Germanium (SiGe) layer by hydrogen implantation induced relaxation. This film is then transferred to glass by direct wafer bonding and hydrogen induced exfoliation. Although part of the SiGe layer is also transferred to glass, due to the high etch selectivity between SiGe and Si, a very smooth Si layer with thickness less than 50 nanometers (nm) can be easily achieved.
The above-described transfer technique makes it possible to develop future advanced devices on inexpensive glass substrates, and advanced display devices can benefit from an improved silicon quality. However, this process require two hydrogen implantation steps, two chemical mechanical polishing (CMP) steps, several film deposition steps, and several furnace annealing steps. First, a relaxed SiGe layer is formed and CMP process performed, followed by H2+ split implantation at 50 keV to 160 keV, with a dose of 2E16 to 6E16/cm2. Subsequent splitting is carried out at 300° C. to 450° C. for 2-3 hours. After wafer splitting, it is necessary to remove the top portion of the Si layer and part of SiGe layer by a dry etch step. Another post dry etch anneal is carried out at 400° C. to 650° C. A fine touch CMP is carried out to remove the roughness resulting from the splitting, followed by a selective etch step to remove the SiGe layer.
Even earlier efforts in transferring Si to glass substrate are the so-called “ion cutting” techniques are disclosed by Cai et al. and Shi et al. These processes implant H ions at 50 keV, with doses ranging from 4E16 to 1E17/cm2, for layer splitting after wafer bonding. However, the process is not suitable for low-cost applications. Wang discloses a device transfer method by backside etching to transfer poly-Si TFTs from Si wafers to glass or plastic substrates. However, single-crystal Si would be preferred.
Gmitter et al. disclose a “lift-off” technique to remove an epitaxial film from a single crystal substrate, which is grown and adhered to a second substrate, by selectively etching away a thin release layer positioned between the epitaxial film and the substrate. In practice, the difficulty in this process is the formation and entrapment of gas, formed as a reaction product of the etch process, within the etched channels. The gas bubbles in the channel prevent or diminish further etching, and cause cracking in the epitaxial film. The process can be modified to cause the edges of the film to curve upwardly as the release layer is etched away, thereby providing means for the escape and out-diffusion of the etch reaction products in the area between the film and the substrate. However, control over forming the curved film edges can be problematic.
Jacobsen et al. and Fan et al. disclose a lift-off technique for use with matrix display systems and light emitting diode displays. Fan et al. disclose a technique of coating materials having different coefficients of expansion onto the epitaxial film layers. The entire structure is brought to a suitable temperature to generate thermal stress between the coating compositions, while the structure is subjected to a release etchant. This stress results in the lift-off of individual thin-film areas supported by the coating.
It would be advantageous if single-crystal Si could be formed on glass using a simplified transfer process.